The present invention relates to an objective lens for recording and/or reproducing of the information of an optical information recording medium, light converging optical system, optical pickup apparatus, and recording and/or reproducing apparatus.
The study and development of a new optical pickup system in which a blue violet semiconductor laser having an oscillation wavelength about 400 nm which is recently put into practical use is used for a light source, is advanced. In this new optical pickup system, when an objective lens whose numerical aperture is increased to about 0.85, is mounted, the recording or reproducing of the higher density information than a CD or DVD is attained. When a specific numerical value is listed, while a recording capacity of the DVD using the objective lens of the numerical aperture 0.60 and a semiconductor laser light source of the oscillation wavelength 650 nm is 4.7 Gbyte/side, when the objective lens of the numerical aperture 0.85 and the blue violet semiconductor laser light source of the oscillation wavelength 400 nm are used, it becomes 25 Gbyte/side.
In the objective lens having a high numerical aperture (particularly, more than NA 0.70), because an inclination angle of a marginal ray of light emitted from the surface of the most information recording surface side is more than 45°, the working distance is apt to decrease as compared to the objective lens having a comparative lower numerical aperture mounted onto the conventional optical pickup apparatus. Accordingly, in the optical pickup apparatus on which an objective lens having a small diameter and high numerical aperture is mounted, there is a problem in which a probability in which the objective lens is brought into contact with the optical information recording medium is large depending on a warp of the optical information recording medium.
In this connection, when a decrease of the wavelength of the laser light source or an increase of the numerical aperture of the objective lens is intended, in the optical pickup apparatus composed of a combination of the comparatively long wavelength laser light source and low numerical aperture objective lens, like as the recording or reproducing of the information is conducted on the conventional optical disk such as the CD or DVD, even when it is an almost negligible problem, it is presumed that the problem becomes prominent.
One of the problems is a problem of an longitudinal chromatic aberration generated in the objective lens due to the variation of a minute oscillation wavelength of the laser light source. In the optical pickup apparatus, because the wavelength of the luminous flux emitted from the semiconductor laser used as the light source is generally mono-chrome, although it is considered that the longitudinal chromatic aberration is not generated in the objective lens, there is a case where a mode hop phenomenon in which the wavelength instantly changes by several nm due to the change of the output, is generated. When the longitudinal chromatic aberration of the objective lens is not corrected, there is a possibility that the light conversing position is changed by the mode hop phenomenon and an error is generated in the recording and/or reproducing of the information. As the wavelength of the light source is reduced, because the changing amount of the light converging position is increased, as the light source of the optical pickup apparatus, when the short wavelength semiconductor laser not larger than the oscillation wavelength 600 nm, particularly the blue violet semiconductor laser of about the oscillation wavelength 400 nm is used, the correction of the longitudinal chromatic aberration generated in the objective lens is necessary.
Further, another problem actualized in the decrease of the wavelength of the laser light source and the increase of the numerical aperture is a variation of the spherical aberration generated in the objective lens by the minute oscillation wavelength variation of the light source. In the optical pickup apparatus, the semiconductor laser used as the light source has a fluctuation between individuals of an about ±10 nm in the oscillation wavelength. When the semiconductor laser having the oscillation wavelength dislocated from the reference wavelength is used for the light source, because the spherical aberration generated in the objective lens is increased as the numerical aperture is increased, the semiconductor laser having the oscillation wavelength dislocated from the reference wavelength can not be used, and the selection of the semiconductor laser used as the light source is necessary.
Further, yet another problem which is actualized in the decrease of the wavelength of the laser light source and the increase of the numerical aperture of the objective lens, is a variation of the spherical aberration of the optical system due to an error of the thickness of the protective layer (called also a [transparent substrate]) of the optical disk. Because the spherical aberration generated due to the La thickness error of the protective layer is generated in proportion to 4-th power of the numerical aperture of the objective lens, as the numerical aperture of the objective lens increases, the influence of the thickness error of the protective layer is increased, thereby, there is a probability that a stable recording or reproducing of the information can not be conducted.